Crab Nebula Flares: Too much ado about not too much? D. Kazanas - - PowerPoint PPT Presentation

Crab Nebula Flares: Too much ado about not too much?

• D. Kazanas

NASA/GSFC

The Crab Nebula Spectrum

S IC

Shock acceleration

Flares

De Jaeger Harding 94

Maximum Shock Acceleration Energy

• The shortest shock acceleration time is that of the Bohm time

scale, i.e. τacc ~ R/c ~κ (γ/B) (κ is a constant involving e, c, me).

• During acceleration, electrons suffer also synchrotron losses on

a time scale τloss ~γ/(dγ/dt) ~ λ (1/B2 γ) (λ is also a constant).

• The maximum electron γ is obtained when τacc ~ τloss i.e for

Bγ2 ~ (λ/κ), and the corresponding synchrotron energy will then be ν ~ Bγ2 ~ (λ/κ) ~ 50 MeV dependent only on e, c, h, me.

• The existence of photons at energies about 10 times larger has

cause sensation and re-evaluation of acceleration processes.

• It turns out that the electrons needed to produce these γ-rays

are fraction of the Crab polar cap voltage.

Magnetospheric balance between electric and magnetic forcesCKF +

_ V E

• At the termination shock, the balance can be

disturbed.

• Bringing the (+) and (-) regions closer could

increase E to have particles close the curcuit.

• These particles could get a fraction of the total

potential voltage across the polar cap.

• This will lead to synchrotron emission at

energies larger than the maximum obtained from shock acceleration.

• Clearly these are not detailed arguments by

any sense.

• However, if correct, imply a correlation

between the polar cap V and the γmax of PWN

(it is not inconsistent with the γ ~ a few x 10^8 of Vela estimated in Pavlov et al. 2003).

• Detailed simulations of this process

(reconnection?) should bear in mind that different field lines correspond to different values of the polar cap V.